1. Field of the Invention
The invention relates to ultrasonic probes used for volumetric scanning and, more particularly, to an ultrasonic imaging probe capable of capturing “on-the-fly” scanning planes in successive positions so as to form a volumetric image representation at a real time frame rate.
2. Related Art
Ultrasonic scanners and probes for acquiring and processing volume information related to biologic tissue or a living body are well known in the art. In the early 1980s, the three-dimensional visualization of human fetus began using systems employing mechanical equipment and basic algorithms for volume rendering. It is believed that these systems are the first 3D imaging systems that use a linear array transducer mounted on a position-sensing arm and capable of providing the system with information regarding the spatial position of the transducer. This approach requires complex mechanical equipment and has now been abandoned by the industry in favor of more advanced 3D concepts including 2D array (matrix array) transducer systems and moving probe systems (employing a motorized linear array transducer).
The matrix array transducer systems are capable of scanning a volume of the region of interest without any movement of the sensing device. In this regard, because the elements of the array (i.e., the pixels) are regularly disposed on the surface of the transducer, a combination of selected pixels, and associated predetermined electronic delay lines for enabling the pixels to be controlled independently, enables focusing of the transducer on any location on the region of interest.
Although the first, matrix array approach definitely provides major advantages in comparison with moving probes, the design and assembly of matrix array transducer devices turns out to be quite intricate, especially in terms of the repeatability and homogeneity of the pixels. In the latter regard, several thousand pixels are necessary to achieve an acceptable image quality. Further, the synthetic acoustic apertures obtained from pixel combinations are of different homogeneities in different orientations. In addition, the required electrical matching of each pixel to the transmission line also presents problems. With regard to the associated imaging system for driving the matrix array transducer, this requires the development of a specific electronic mainframe providing a very large number of independent channels. Because of the high number of pixels to be controlled, the resultant mainframe design specifications rapidly push the limits of the available technologies in terms of component performance and miniaturization so that, currently, the matrix-based imaging systems are typically dedicated to research activities where issues of costs and intrinsic image quality can be balanced by scientific interests and educational considerations.
The second group of ultrasound instruments, i.e., the so called “moving probe” instruments, is theoretically more basic and can be considered to be an extension of the well known linear array transducer technology that operates on conventional or existing imaging systems. The ultrasound transducer used is very similar to conventional devices designed for 2D scanning operations, such as flat or curved linear arrays or phased arrays well known in the art. The transducer head of these instruments is mounted in a moving probe that also includes a mechanism for the transducer assembly including guiding and motorization means for moving the transducer probe during scanning operations. Since the probe uses a linear array transducer, imaging systems compatible with such a probe can be of conventional types and designs that are widely available commercially, and wherein minor upgrades have been implemented such as the addition of an add-on transducer interface and volume rendering software.
In theory, 3D scanning using a linear array transducer can be performed through transducer movements such as swinging, sliding or rotating. The movement can be an alternating movement (e.g., forward and backward) or continuous, and the amplitude of the transducer movement can extend over a range of from a few degrees to 180 degrees or more, depending on the requirements of the particular application. Because, in general, the construction of a 3D moving probe only requires an array transducer coupled to a suitable motorization means and sealed by a probe housing providing an acoustic window for enabling energy transmission to and from the region of interest, a number of different methods and devices are disclosed in the prior art that are usable for 3D scanning. However, in general, two transducer movements are preferred, viz., a rotational movement and a swinging movement.
More specifically, the rotating devices basically comprise phased array probes wherein the transducer footprint is small and the scanning geometry is of sector shape. It is noted that this group of probes is particularly well adapted for cardiology wherein a small emitting surface and sector scanning are required.
On the other hand, swinging probes have larger acoustic windows in order to carry out operations using a linear array transducer. With this type of probe, a synthetic image is obtained from the electronic driving of the elements of the array and, therefore, volume information is obtained by swinging of the array transducer in the elevation plane. This volume scanning method has the advantage of minimizing any encumbrance on the probe operation, and is capable of providing volume images of most human organs without moving the probe itself.
In practice, rotating and swinging probe devices are characterized by different design and assembly difficulties. For example, insofar as the moving transducer of the probe is required to be immersed in a coupling fluid so as to enable acoustic signals to be effectively received through the probe windows, the motorization means for the transducer must be assembled separately from the coupling chamber. This is done to prevent the fluid from causing problems with the operation of the motor and/or transmission. In the rotating devices, providing sealing of the motor from the transducer is quite straightforward. In this regard, an O-ring seal is usually disposed on the output shaft of the motor at the boundary thereof with the liquid chamber so as to provide liquid sealing of the motor.
On the other hand, with respect to swinging probe devices wherein a linear array is caused to laterally swing through an arc, the transducer is mounted on a swinging mechanism having an axis of rotation that is aligned with the azimuthal plane of the transducer. The rotation axis is then connected to the motor output so as to provide the desired swinging movement of the transducer. The coupling provided between the motor and the swinging mechanism can be provided by direct coupling, a pulley system or an intermediate gearbox. Since the rotation axis is azimuthally aligned, the separation between the wet chamber (i.e., the portion of the housing containing the coupling fluid) and the exterior becomes significantly greater than with previous devices.
Methods and devices for implementing 3D ultrasonic probes that have been disclosed in the prior art will be further described below to demonstrate the advantages and drawbacks of these prior art methods and devices.
For example, mechanically moving ultrasonic 3D/4D probes of interest are disclosed in U.S. Pat. No. 5,159,931 to Pini which relates to an apparatus for three-dimensional reconstruction of anatomic body. The apparatus comprises an ultrasonic mechanical probe wherein a sector scanning transducer is moved through an angle of 180 degrees in order to form a volume image. Rotation of the scanning plane can be achieved either mechanically, using a motor for rotating the transducer, or electronically, using a matrix transducer. The method of forming a 3D image is disclosed in the patent but no details are provided regarding the mechanical probe construction..
U.S. Pat. No. 5,152,294 to Mochizuki discloses a swinging ultrasonic apparatus for three-dimensional imaging. In this apparatus, a probe housing or casing receives a swinging mechanism for an ultrasonic array transducer, and a motorization means is provided for moving the transducer. A position coding device is used for sensing the transducer position during the operation thereof and for supplying this information to provide control of the system. The apparatus also comprises external shell that isolates the surface of the transducer from the body to be scanned. A coupling liquid is used for enabling the transmission of acoustic energy through the housing shell. A partition membrane is provided between a transducer end portion and the housing or case so as to hermetically retain the fluid coupling inside of the housing. In order to avoid any internal reflections of traveling echoes, the internal curvature of the housing or case is made larger than that of the transducer so as to direct reflected echoes to the bottom side of the housing or case at which an absorbent material is disposed.
The apparatus of the Mochizuki patent suffers important shortcomings with respect to the curvatures of the transducer and the case. In this regard, the non-uniform distance separating the transducer from the case shell strongly affects the homogeneity of the probe output. The amount of attenuation varies with the distance and the resultant impedance mismatches (sound speed and density) produce deviations in the transducer radiation pattern which contribute to an increase in side lobe levels and to focal aberrations. Further, the liquid partition membrane, which is provided as an add-on device over the front surface of the transducer, causes image artifacts and produces attenuation of the acoustic energy.
In the Mochizuki patent, the use of a loose membrane is proposed so as to allow the transducer to be swung with ease. The associated drawings indicate a “S” shaped membrane is provided for this purpose. As described, the partition membrane of the Mochizuki patent is compressed and released laterally aside from the transducer, during operation, thereby introducing a constraining force when the transducer is swung. This constraining force will result in non-uniform transducer movement so that the overall system must be implemented with a real time position correction algorithm. While a probe apparatus is well adapted for a conventional 3D imaging process wherein the transducer is incrementally stopped in a first position to effect acquisition of an image and then is stepped up to the next position to effect acquisition of a further image, this method is not, however, compatible with “on-the-fly” 3D imaging processing wherein the volumetric information is instantly available during probe operation and wherein the transducer movement must be accurately correlated with the images being acquired.
Turning to the available methods for providing liquid separation in mechanical ultrasonic devices, the use of a flexible membrane for sealing the coupling liquid bath against the external environment has been disclosed in the prior art at least since 1984. In this respect, Biophysic Medical, France proposed at that time a A-scan/B-scan ophthalmic scanner commercially designated as Ophtascan™. The scanner is typically equipped with a “B-mode” imaging mechanical probe so as to provide users with sector viewing of the organ. Probes that are compatible with such systems include a swinging transducer which is moved by motorization means connected thereto through a rotating/swinging movement transformation system. The transducer is equipped with a flexible membrane which is sealed, respectively, to the periphery of the transducer and to the probe housing, in order to provide separation of the fluid chamber. The flexible membrane is made from an elastomeric material (e.g., latex, silicon rubber, natural caoutchouc or the like) having a wave shape from the center to the periphery. The center of the membrane is provided with a hole that matches the transducer circumference and is bonded to that circumference. The periphery of the membrane is attached or sealed to the probe housing. When assembled, the transducer is capable of swinging movement over the operational amplitude thereof while compressing and releasing the membrane portion in the direction of movement.
The scanner proposed by Biophysic Medical is designed for moving single element devices wherein the diameter of the transducer is small enough, as compared with the dimensions of the membrane, to allow a swinging movement of the transducer within acceptable mechanical constraints. Because the membrane is mounted horizontally and secured to the diameter of the transducer, the mechanical effort that must be exerted to overcome the mechanical constraints varies with the position of the transducer. As a result, it is necessary to make corresponding corrections in the motor and in system operations.
Major drawbacks suffered by the Biophysic Medical scanner apparatus include, first, the assembly method required for attaching the membrane to the transducer case is unreliable and the resultant membrane attachment is subject to leaking of the fluid. Second, the required positioning of the membrane increases the effort that must be provided by the motor when the transducer reaches the extreme positions of the swinging movement thereof. Moreover, these observations are also relevant to the apparatus disclosed in the Mochozuki patent wherein a similar membrane is assembled and used therein..
With respect to the prior art discussed above and the method of making 3D ultrasonic probes herein described, there is obviously a need to improve existing 3D probe constructions and manufacturing methods in order to provide better reliability and performance.